Most active noise control systems utilize acoustic drivers in conjunction with acoustic sensors, controller(s) and associated signal conditioning electronics to reduce preselected sound pressure levels from impinging upon the ear drum. The instant invention is in the form of a personal system which may take the form of a headset, a "silent seat"(one designed to attenuate sound pressures at the users ears when the user is occupying the chair) or other form of personal quieting system. For example, the instant system can be employed as part of the headliner in an automobile for the purpose of attenuating road, engine or other designated noise. The instant invention overcomes the current limitations of existing devices by the use of spatial adaptation of an acoustic error sensor and implementation of a unique heteronomous control algorithm. Additionally, the user has increased comfort in the headset configuration by use of non-contacting electroacoustic transducers.
The field of active noise cancellation has progressed from the simple attempts in the 1970s by Chaplin in the United Kingdom to attenuate noise to todays more complex systems which are geared to specific types of noises. The field of noise cancellation has been reviewed extensively in "Active Control of Sound" by P. A . Nelson and S. J. Elliot, Academic Press, 1991. Progress in attenuating tonal noise has included the development of digital virtual earth systems which use fewer sensors than heretofore employed (see U.S. Pat. No. 5,105,377 to Ziegler et al entitled "Digital Virtual Earth Active Cancellation System". Cancellation of unwanted broadband noise has seen development of adaptive feedforward systems which measure the noise prior to its arrival at the cancellation point. In some applications these systems have been combined to attenuate a mixture of objectionable noises. By the use of frequency domain algorithms control over the characteristics of the noise cancellation has been achieved and these algorithms have been further modified by harmonic filters in constant rate sampling of sound converting time domain signals into frequency domain signals (see U.S. Pat. No. 5,361,303 to Eatwell entitled "Frequency Domain Adaptive Control System"). Adaptive speech filters have enhanced all of the prior art attempts at noise attenuation and/or cancellation by measuring the spectrum of the data and blocking any frequencies that do not exhibit statistical properties of standard speech thereby allowing speech in noisy environments.
The use of adaptive filtering techniques is widespread today and characterized by the controller characteristics being adjusted according to an algorithm such as that disclosed by Widrow and Stearns, "Adaptive Signal Processing", Prentice Hall, 1985. Both feedback systems (see U.S. Pat. No. 4,494,074 to Bose entitled "Feedback Control") and feedforward systems (see U.S. Pat. Nos. 4,122,303 and 4,654,871, both to Chaplin and U.S. Pat. No. 4,878,188 to Ziegler) have been used before in personal quieting systems. Adaptive filtering techniques are discussed in the patents to Graupe (has U.S. Pat. No. 5,097,510) and Graupe and et al (U.S. Pat. No. 4,025,721).
Despite the large amount of development in the personal quieting system area, the instant invention has not been conceived of by others in the field. No one heretofore has shown or described the simultaneous use of feedback and adaptive signal processing algorithms (heteronomous control) to target different features of the noise field. Nor are there any prior patents or disclosures describing the use of a spatially adaptable error microphone based on the changing dimensions of the silent zone in different noise fields.
It has been suggested to incorporate both asynchronous feedback and microphone-based feedback compensation cancellation techniques into a single system. The attenuation concept discussed by Casalli (J. G. Casalli and G. S. Robinson, "Narrow-Band Digital Active Noise Reduction include In a Siren-Cancelling Headset: Real-Ear and Acoustical Manikin Insertion Loss", Noise Control Engineering Journal, 42 (3), 1994, May/June., page 101.) but no system has been built or developed. Casalli refers to a siren-canceling headset not unlike the one described in U.S. Pat. No. 5,375,174 to Denenberg entitled "Remote Siren Headset" which is hereby incorporated by reference herein. The architecture that the article suggests is totally different from that of the instant invention and nowhere in the article does it suggest adaptive positioning of the noise microphone. There is no discussion in the article or elsewhere of using a remote microphone for a blended feedforward/feedback architecture.
There have been endless variations on the noise cancelling headset over the years including those disclosed by Wadsworth in U.S. Pat. No. 3,098,121, Chaplin et al, in U.S. Pat. No. 4,654,871, Twiney et al, in U.S. Pat. No. re 4,953,217, Bourk in U.S. Pat. No. 5,182,774 and Nishimoto et al, in U.S. Pat. No. 5,402,497, all of which are hereby incorporated by reference herein. The use of circumaural headsets dominates the ANR headset market due to the lower actuator demand in the quiet enclosure afforded by earmuffs. While there are supraural headsets the instant device differentiates from them by being open-air thus affording no confinement whatsoever of the user's ears. The open air system requires controlling a higher level of sound pressure and wider variance as there is no confinement by the muffs, whether supraural or circumaural.
Various systems to affix earpieces to headgear have been proposed which those shown in U.S. Patents to Altman and Goldfarb et al, U.S. Pat. Nos. 5,329,592 and 4,682,363, respectively, both of which are hereby incorporated by reference herein.
Remote control of headsets has been suggested as evidenced by U.S. Patents to Schwab and Hsiao-Chung Lee, U.S. Pat. Nos. 4,845,751 and 4,930,148, respectively.
A review of the current status of active noise control headsets illustrates the advantages of the invention. The vast majority of active noise headsets employ either feedback compensation, as in the Bose et al patent, or adaptive signal processing algorithms, as described in U.S. Pat. No. 5,375,174 to Denenberg, implemented in time domain or frequency domain format. These two distinctive architectures have unique characteristics especially in relation to one another. Feedback control relies on a compensator to maximize the sensitivity function within the stability bounds specific to the particular noise field under consideration and active noise hardware in use. This arrangement results in a reduction in the closed-loop, low frequency gain between the disturbance input (the surrounding noise field) and the output signal (the error microphone). Noise relief realized by this technique is typically between 15 to 20 dB re 20 microPa and can be achieved from approximately 50 to 700 Hz. These limitations on noise reduction and performance bandwidth cannot be overcome for reasons that are documented by experts in the active acoustic control community. In this regard see also U.S. Pat. No. 5,251,263 to Andrea et al, entitled "Adaptive Noise Cancellation and Speech Enhancement System and Apparatus Therefore". Adaptive feedforward noise reduction for personal ANR systems has also been proposed but to a much lesser extent. Such an architecture relies on the availability of a reference signal which is correlated with the estimate of the noise field and cannot be destabilized by the control signal. Such references have been constructed for the case of periodic inputs (see Chaplin et al) such as a reciprocating pump or propeller which can be used to spawn synchronous reference signals which serve as inputs to the adaptive filter . The other approach is to provide a compensator which cancels the feedback path between a so-called controllable reference signal and the control signal, e.g., the filtered-u algorithm. The degree of noise suppression for adaptive feedforward systems is a direct function of the multiple coherence (between the constructed, or otherwise available, reference signal and the acoustic sensor which will be minimized) EQU dB reduction=10 log.sub.10 (1-.gamma..sup.2)
The performance bandwidth is limited by the sampling frequency for the digital filter and the size of the adaptive filter but can practically achieve noise reductions into the kHz range. Theoretically, this approach can provide up to 50 dB suppression of noise levels and more than triple the feedback control bandwidth of the feedback methods.
The architecture of the essential components in any personal ANR system also has profound influence on the absolute and user-perceived performance of the system. Existing active noise control headsets and systems are designed using fixed spatial separations between the electroacoustic transducers and the acoustic sensor near the listeners ear(s). Recent theoretical and experimental results have proven that the spatial dimension of the noise field reductions is a nonlinear function of the noise frequency, the electroacoustic transducer, and the separation distance between an electroacoustic transducer surface and the acoustic sensor being controlled. The silent zone spatial dimension is relatively small for typical headset components/geometries and varies with the noise frequency (FIG. 1). For a fixed frequency, the silent zone dimension varies with separation distance between the acoustic sensor and the driver (FIG. 2). This variability of the silent zone's spatial and temporal characteristics has not been properly exploited in any existing designs for personal ANR systems.
The prior art in personal ANR technology has reached an impass imposed by the tradeoffs which currently exist for the available architectures. Feedback control headsets can provide robust noise reductions, nominally 15 dB from 50 Hz to 700 Hz, but do not require the identification or generation of an uncontrollable reference signal. Adaptive feedforward headsets can achieve substantially higher noise reductions, particularly at tonal disturbances, but must have a correlated, uncontrollable reference signal available. Both types use fixed relative positioning between the electroacoustic driver, the acoustic error sensors, and the listener's eardrum. More specifically, the prior art fails to combine the features of both architectures in a single personal ANR system and fails to exploit the nonlinear dependencies of the silent zone created around by the suppression of a single error microphone. Headsets produced in the past such as the "Proactive" and "Noisebuster" headsets of Noise Cancellation Technologies, Inc. as well as those of Sennheiser, David Clark and Bose fail to contemplate the features constituting this invention.
While all the prior art discussed above relates to personal ANR systems, they are limited by lack of performance in noise fields dominated by broadband and tonal disturbances. Furthermore, they fail to optimize the perceived effectiveness, as perceived by the user, by providing real-time or psuedo real-time adaptation of the relative positioning of the ANR components. Therefore, the following invention embodies heteronomous control and adaptive spatial positioning of the ANR components, along with an open air arrangement so as to surpass the prior art in performance and comfort for the user.